The present invention relates to a control method for a cordless telephone system for connecting a plurality of stationary apparatuses and a plurality of movable portable telephones through radio channels and, more particularly, to an extension call among the portable telephones.
FIG. 1 shows an arrangement of a cordless telephone system. In FIG. 1, reference numeral 1 denotes a main apparatus connected to a plurality of telephone lines and having a line connection function; 10.sub.l to 10.sub.n, stationary apparatuses each having a radio transmitter/receiver; 20.sub.l to 20.sub.m, portable telephones; and 30.sub.l to 30.sub.n, radio zones within which the portable telephones 20.sub.l to 20.sub.m can perform communication.
The radio zones 30.sub.l to 30.sub.n respectively correspond to the stationary apparatuses 10.sub.l to 10.sub.n. The portable telephones 20.sub.l to 20.sub.m can be connected to a corresponding one of the stationary apparatuses from anywhere within the assigned radio zones 30.sub.l to 30.sub.n, and can perform an office line outgoing/incoming call, an extension outgoing/incoming call, and the like.
In the cordless telephone system with the above arrangement, however, although one portable telephone can call all other portable telephones, the calling portable telephone cannot detect whether or not other portable telephones perform an incoming call display. This will be exemplified below.
FIG. 2 shows a conventional control method of a cordless telephone. In this case, connection procedures of simultaneous calling are illustrated, i.e., when a caller in just a single calling operation calls multiple destinations. For the sake of simplicity, the system is assumed to be constituted by a main apparatus, first to third stationary apparatuses, and first to third portable telephones. Assume that the first to third telephones belong to radio zones of the first to third stationary apparatuses, respectively. Furthermore, the second portable telephone is connected to the second stationary apparatus through a radio channel and is busy through an office line.
In FIG. 2, the first portable telephone sends a call request signal S.sub.21 to the first stationary apparatus in response to a simultaneous calling operation by a user. Upon reception of the call request signal S.sub.21, the first stationary apparatus transmits a call request signal S.sub.22 to the main apparatus. When the main apparatus receives the call request signal S.sub.22, it transmits an incoming call signal S.sub.23 to the third stationary apparatus which is not in use (the second stationary apparatus is in use). Upon reception of the incoming call signal S.sub.23, the third stationary apparatus transmits an incoming call signal S.sub.24 to the third portable telephone. The third portable telephone performs an incoming call display upon reception of the incoming call signal S.sub.24, and informs a user of the third portable telephone of simultaneous calling.
However, although the first portable telephone performs simultaneous calling to the second and third portable telephones, the second portable telephone (not shown) is busy through an office line, and only the third portable telephone performs an incoming call display. Meanwhile, the first portable telephone cannot detect whether or not other portable telephones perform the incoming call display. Thus, a user of the first portable telephone makes a wrong judgement that the second portable telephone performs the incoming call display.